Differential regulation of the human H1 zero-histone-gene transcription in human tumor-cell lines.

Cloning and sequence analysis of about 2 kb of the 5' flanking region of the human H1 zero histone gene reveals several potential regulatory elements upstream of the transcribed portion of this gene. Transfection studies using the chloramphenicol acetyl transferase (CAT) gene as a reporter gene with a series of promoter deletions revealed that the expression of the H1 zero gene may depend on a complex interplay of several transcription factors, including members of the retinoic acid and/or thyroid-hormone-receptor superfamily, at the 5' flanking region of the H1 zero gene. CAT assays demonstrate varied patterns of expression and regulation in different human tumor-cell lines. The leukemia cell line HL60 does not express H1 zero mRNA and shows no CAT activity. HeLa cells strongly express the CAT gene under the control of the H1 zero promoter. Under the same conditions, HepG2 cells also transcribe the CAT gene, although at a lower rate than HeLa cells. Using different promoter-deletion clones, the CAT activity differs in HepG2 and HeLa cells in the very distal promoter region. In both cell lines, the CAT activity decreases several fold when the region between nucleotides -450 and -600 upstream of the mRNA start site is deleted. It also decreases when just the proximal portion but not the distal promoter region is deleted. In summary, the regulatory patterns of these three cell lines differ, indicating a cell-type-specific regulation of the human H1 zero-histone-gene expression.